This invention relates generally to high-power laser systems, and more particularly, to optical systems for alleviating the effects of atmospheric conditions on high-power laser beams. When a high-power laser beam is propagated through the atmosphere, its coherency, and therefore its power, are degraded by a number of uncontrollable atmospheric effects. One of the most significant effects is thermal blooming, which occurs when the beam energy is non-uniformly absorbed by components of the atmosphere, such as water vapor. This absorption process gives rise to density distortions in the atmosphere, and a planar wavefront propagated from a laser can then be subject to substantial aberrations. Wavefront distortion greatly reduces the peak power of the beam, and although various approaches have been suggested to alleviate the problem, none is completely satisfactory.
One of the simplest approaches is to increase the diameter of the exit aperture through which the beam is propagated. Studies have indicated that the use of large exit apertures diminishes the effects of thermal blooming. However, the use of large apertures, in excess of one meter in diameter, has some practical limitations, such as high fabrication cost and thermomechanical stability of the components.
Another technique for reducing the effects of thermal blooming is to employ principles of adaptive optics. Light reflected from a target on which the beam impinges is analyzed to determine the profile of wavefronts after atmospheric distortion. Then a deformable mirror can be employed to apply compensating distortions to the transmitted wavefronts. This process of wavefront reconstruction is technically complex and expensive. More importantly, the technique works well only in the correction of distortions that occur relatively close to the source of the beam. When atmospheric distortions occur at relatively long distances from the source, the adaptive technique is less successful.
It will be appreciated from the foregoing that there is a need for a simpler technique to reduce the effects of thermal blooming on high-power laser beams. The present invention is directed to this end.